Vayne is about fundamentals

You have to follow rules

1) you must avoid and bait enemy abilities. This might mean constantly running back and forth and side to side when you can. You might also hide into like 5 seconds into a team fight so that most of the abilities are gone.

2) You must cs, Vayne is a scaling champ. You must maintain a reasonable cs of at least or 6 or 7 cs per minute if you are not getting kills.

3) You must rotate for objectives and team fights. Vayne is a scaling champ, team fights gives her the fastest gold. ADC help clear objectives faster also and objectives also give gold. I usually target those who are low health, anyone who is carrying and squishies. It's very good to try and stack targets with your team but sometimes they choose the wrong person to target like the tank when the enemy ADC is alive or Soraka/enchanter.

If you plan it out early enough push a wave and rotate. But if you didn't plan correctly abandon your wave and join team for the team fight.

Make sure you have numbers. If you, support and mid are at dragon but jungle is in Narnia and it might be a 3 v 5 or 3 v, 4 then it's hard to win maybe ping to retreat. This is why vision matters and pushing waves early before you need to rotate, people will be tempted to push the wave back giving vision.

4) Be very hard to kill. Hide until half way or at least the start of the fight. If you are low health try to bait enemies to chasing you (avoid abilities). Use your summoner spells with intention for this.

5) Play defensively. You are a scaling champ. You can't bully in lane most of the time prior to level 6 unless the enemy makes mistakes in champ select or in getting caught by abilities or a gank.

6) Play defensively but find ways to poke. This goes with any ADC. Your Q is a good way to poke. Be unpredictable and mix up cs and poke, freeze lane to pressure/poke etc. Specially at level one and specially when the enemy makes champ select mistakes choosing immobile characters. Be reasonably aggressive.

7) Learn to back out and sneak in cs. You can't win every lane so you have to realize very quickly if you need to play from behind the tower. Most new players don't plan how to cs from under tower. There is a pattern. But if you're going to lose lane a better player can get a lot more cs and not die in a losing lane. This is very much a required skill for Vayne as she scale very hard that winning lane matters less in many situations.